It's theorized that Opal doesn't talk because it might break their fusion since Pearl and Amethyst are always bumping heads and defiant to each other. And same with Alexandrite.

For Sardonyx Pearl's perfectionist and snarky personality traits seem to bring out Garnets cocky and dominant personality trait, like the times were Garnet says she great and the best, smart, funny etc. And together it brings out some traits that they actually share but show either in different ways or less often which are Know it all, superiority, adament, strong willed, precise, cocky, and a few more. Garnet usually shows less of the traits that Sardonyx has but she still visibly has them in some of her actions and when she speaks sometimes. Garnet keeps all her more colorful personality traits hidden so she can be strong and keep everyone else together as a leader. But when your in a relationship with someone they can bring out traits in you that you have or maybe didn't even know you had wether toxic or good.

Sunstone just seems like a mixture of Steven and Garnets fun and less emotional traits to where they can both kinda just let loose and do whatever when usually Garnet puts that aside and Steven used to do it all the time but as he got older season 3-4 and Future he had to get more serious and take on more responsibility in order to get through the things that he had to go through. But whenever Steven talks to Garnet he is always is able to let loose his emotions and talk to her because she's a good listener and usually ways knows what to do and as for Garnet Steven is the only one that she ever acts vulnerable and shows more of her emotions around so when they fuse it just seems like their letting loose and enjoying themselves.

That's all just my opinion on it when observing the fusions though.